1. Field of the Invention
The invention relates to a two-step process for coloring aluminum electrolytically with metal salts using a direct current in an acidic solution to produce a defined oxide layer and then coloring the layer using an alternating current in an acidic electrolyte which contains tin(II) salts.
2. Description of the Prior Art
According to the state of the art, processes for coloring anodized aluminum are divided into chemical processes and electrolytic processes.
Chemical processes for coloring involve coloring of anodized aluminum in an aqueous phase with suitable organic and/or inorganic compounds without the use of a current. Electrolytic coloring may be carried out in one or two steps.
For the one-step electrolytic process, aluminum is generally anodized as well as colored in a single step using direct current in an electrolyte composed of sulfuric acid in admixture with suitable organic acids, such as, for example, maleic acid, oxalic acid, sulfosalicylic acid or sulfophthalic acid.
With the two-step electrolytic process, the so-called electrolytic coloring with metal salts, a defined oxide layer is produced initially in a first process step using direct current with sulfuric acid or sulfuric acid and oxalic acid as the electrolyte liquid. The thus anodized aluminum is then colored electrolytically in a second process step, using alternating current and solutions of certain metal salts or mixtures of metal salts.
The electrolytic process is used predominantly for coloring aluminum because it produces higher light stability in the color and is more efficient economically. Because of the lower costs involved, electrolytic coloring with metal salts is clearly the predominant electrolytic process. In this process, solutions containing tin(II) sulfate are preferably used.
In the two-step process, when using solutions containing tin(II) sulfate for electrolytically coloring aluminum, shades of color can be produced which, depending on the operating conditions chosen, range from silvery, through light, medium or dark bronze, to black and which are resistant to light and to normal atmospheric effects. As a result of hydrolysis and/or oxidation, however, sulfuric acid solutions of tin(II) sulfate, of the type used for the electrolytic coloring of aluminum with metal salts, deposit, to an increasing degree depending on their age, difficulty soluble tin compounds which are ineffective for coloring aluminum.
Attempts have been made to reduce the deposition of such difficulty soluble tin compounds by the addition of suitable compounds, such as, for example, phenolsulfonic acid, cresolsulfonic acid, phenol or its derivatives. Because of their harmful effects on effluent water, their toxicity as well as their odor, these compounds are now classified as environmental contaminants. Additionally, such compounds do not adequately suppress the formation of difficulty soluble tin compounds.